Control and protection arrangement for a d.c. power transmission system

ABSTRACT

A current from a power transmission line is smoothed by a filter and then compared with a set value to detect and control the transmission current when the transmission current is in the neighborhood of the setpoint. Whereas, the current is directly detected and controlled to the set value without intervening the filter when the deviation rapidly increases.

United States Patent Machida et al. 1 Apr. 25, 1972 CONTROL ANDPROTECTION {56] References Cited ARRANGEMENT FOR A D.C. POWER N T PA TTRANSMISSION SYSTEM U TEN S 3,487,286 12 1969 P ..321 2 [72] Inventors:Takehiko Machida; Yukio Yoshida, both 3 513 353 5x970 ersson of Tokyo;Koji Iwata; Keniiro Yokoyama, 353559i /1970 both of Hitachi-shi, all ofJapan 3383579 5/1968 [73] Assignees: Hitachi Ltd., Tokyo, Japan; ZaidanHojin 3,444,453 5/1969 Denryoku Chuo Kenkyusho, Tokyo, Japan ,4 7/ 19693,501,685 3/1970 Reese et a1 ..321/2 [22] Filed: Aug. 28, 1969 2 App]353 3 Primary Examiner-William H. Beha, Jr.

Att0rneyCraig, Antonelli, and Hill Foreign Application Priority Data 57ABSTRACT Sept. 4, 1968 Japan ..43/63021 A sun-em f a power transmissioni is smoothed by a filter and then compared with a set value to detectand control [52] US. Cl ..32l/14, 317/31, 321/2, the transmission sun-emwhen the transmission current is in 321/ the neighborhood of thesetpoint. Whereas, the current is [51] Int. Cl .H02m 11/18, H02p 7/ 14directly detected and controlled to the set value i h i [58] Field ofSearch ..323/4, 9; 321/2, 1 1-14, vening the fil when the deviationrapidiy increases 4 Claims, 6 Drawing Figures AVKU r 9 FILTER /3 AMPVwfi 6/5 /9 27 Q /6 24 P f E AMP .1. M Q, SHIFTER g 26 i i a 14/7 30 [EU3/ /7 2 25 AMP 6Z0 3/ 6/7 Patented A ril 25, 1972 3 Sheets-Sheet l F/G.PRIOR ART 4005/? CONTROLLED SYSTEM \3 5,

2 co/vmom/va SYSTEM F/G. .2 PRIOR ART INVENTORS TAKEHIK MA H- DA Yuk E0yes/110A,

ATTORNEY Patented A ril 25, 1972 3 Sheets-Sheet 3 FIG. 4b

FIG. 4a

FIG. 5

T/ME

INVENTORS V0 5 H [DA IAKEHIKO MACH EDA yu K120 KDJI .EWATA 4nd KENJ IR YK ATTORNEYS CONTROL AND PROTECTION ARRANGEMENT FOR A DC. POWERTRANSMISSION SYSTEM In automatic control systems, it is naturallypreferable that the temporary value of a controlled variable alwayscoincide with the setpoint. It is also required at the same time thatthe automatic control system should be stable, i.e. the automaticcontrol'system should have little response to minute deviations,especially ripples. For this reason, the detection of a controlledvariable is usually done through a filter in automatic control systems.

When a filter is used in the detection of a controlled variable,however, it gives some delay in the response of the control systemdepending on the time constant. Such delay may sometimes allow thecontrolled variable to go beyond the permissible value.

As an example, a d.c. power transmission system will be describedhereinafter. In a d.c. power transmission, system for example, thetransmission current is controlled to a constant value on the rectifierside. In this case, the current flowing through a transmission line isdetected and compared with the setpoint to control the rectifier by thedeviation from the setpoint. But, since a transmitted current usuallyhas ripple components, detection is done through a filter to avoid theinfluence of these ripple components. The time constant of such a filteris of the order of 100 to 200 milli-seconds. Thus, if there occurs sucha commutation failure of the power inverter that allows an excessivecurrent to suddenly flow through the transmission line, it willnaturally take about 100 to 200 milliseconds for the control system todetect it and respond to the excessive current. That is, before thecontrol system begins to work, an excessive current flows for several toten and several Hz of ac. power. When the rectifier is formed by athyristor, an excessive current for such time is enough to cause breakdown. Therefore, it is necessary to provide a separate protection meansto cut off such an excessive transmission current to protect therectifier. But if such an excessive current can be controlled to anormal value at most in 1 Hz., there is no need to stop the transmissionand when the inverter has recovered to a normal state, then normaltransmission will continue.

As is described above, it is required for an automatic control system tobe quickly responsive to excessive deviations as well as to be stableagainst minute deviations.

This invention is designed to satisfy such a requirement and provides acontrol and protection system for a d.c. power transmission systemcomprising a usual feedback loop and another feedback loop having littletime delay. This invention also provides a control and protection systemparticularly useful for d.c. power transmission.

According to one feature of this invention, controlling action beginsimmediately after the controlled variable goes beyond the upper or thelower limit of the setpoint.

Other features and advantages of this invention will be made clear inthe following description made in connection with the accompanyingdrawings in which:

FIG. 1 is a block diagram illustrating the structure of a conventionalautomatic control system;

FIG. 2 is a block diagram illustrating the conventional constant currentcontrol of a power rectifier in a d.c. power transmission system;

FIG. 3 is a block diagram showing the main part of an embodiment of thisinvention;

FIGS. 4a and 4b illustrate the input-output characteristics of theamplifier and the phase shifter of an embodiment of this invention; and

FIG. 5 illustrates the manner of control of this invention in comparisonwith that of a conventional system.

FIG. 1 illustrates the construction of a conventional automatic controlsystem comprising a controlled system 1 including machine, process, orthe like, and a feedback system 2 including detector, filter, amplifier,or the like. Reference characters G, and G designate the transmissionfunction of the systems 1 and 2, respectively. The output e, of thefeedback system 2 is compared with the setpoint e at an adder 3 to givethe deviation Ae =e e, as an output to the controlled system 1. Thecontrolled value e of the system 1 is derived by the deviation A e. Theoutput e, of the feedback system 2 is obtained by the measurement ofcontrolled value e The control system is so designed that the deviationAe is always zero. But since the transmission functions G and G usuallyhave time constants T and I of some magnitude, respectively, thepreviously mentioned excessive current arises especially when T islarger than T FIG. 2 is a block diagram for illustrating the constantcurrent control of a rectifier in a d.c. power transmission system. Thesystem comprises two a.c. systems 4 and 5, a rectifier 6 and an inverter7 connected to the ac. systems 4 and 5 respectively, transmission lines8 and 9 between the rectifier 6 and the inverter 7, a d.c. currentdetector 11 for detecting the current L, flowing through thetransmission line 9, a filter 13 for removing ripple components from theoutput of the detector 11, a setter 14 for providing a setpoint, anadder 15 for comparing the output I, of the filter 13 with the set valueI and an automatic pulse phase shifter (APPS) 12 for controlling thepower rectifier 6. When this unit is operated to transmit electric powerfrom the system 4 to the system 5, the power rectifier 6 works as ana.c.-to-d.c. converter and the inverter 7 works as a d.c.-to-a.c.converter. The automatic pulse phase shifter 12 receives the output A!1,, 1 of the adder l5 and controls the phase of the gate pulses for therectifier 6 to make the deviation AI zero. In the normal state, theinput I, and the output 1 of the filter 13 corresponds to each other andthere arises no problem. But when the transmission current I, issubjected to a sudden and large change and if the time constant of thefeedback system including the detector 11 and the filter 13 isrelatively large, there arises a possibility that the rectifier may bebroken down by a large current.

FIG. 3 shows the main portion of an embodiment of the invention in blockdiagram, which has eliminated the above drawback, in which similarreference numerals indicate the similar parts as those of FIGS. 1 and 2.In this embodiment, two subordinate feedback systems comprising addersl6 and 17, amplifiers 19 and 20, and diodes 24 and 25 are connectedbetween the detector 11 and the automatic pulse phase shifter 27 inparallel relationship with the main feedback system comprising a filter13, an adder 15, an amplifier 18 and a diode 23. It is to be noted thatthe subordinate feedback systems contain no filter. The adder 16 issupplied with a setpoint of I ,B and the output of the detector 11 andthe adder 17 with a setpoint of I 'y and the output of the detector 11,where B and y are positive. The polarities of these inputs are indicatedin the drawing. The amplifiers 18, 19 and 20 give a certain outputs whenthe output of the adders 15, 16 and 17 have a certain positive value A Ibut give no or little output when the output of the adder is smallerthan Al including negative values. Resistors 21 and 22 are inserted inthe output circuit of the amplifiers l8 and 20. The diodes 23, 24 and 25are provided to selectively derive the output of the amplifiers 18, 19and 20. The diodes 23 and 25 are connected in the same polarity and thediode 24 in an opposite polarity. The three feedback system areconnected together at an interconnection point 26 and then to anautomatic pulse phase shifter 27. The phase shifter 27 receives thevoltage at the interconnection point 26 and drives the rectifier with agate pulse having a phase in accordance with the voltage at the point26. A positive power source line 28, a resistor 29, a backward currentpreventing diode 30, and a voltage source 31 for positive biasing areconnected to the point 26, as is shown in the figure, to constitute anauxiliary circuit for affording a necessary voltage at the point 26.FIGS. 4a and 4b show the input-output characteristics of the amplifiers18, 19 and 20 and the automatic pulse phase shifter 27 Thesecharacteristics are not restrictive but preferrable ones for the circuitshown in FIG. 3. The input of an amplifier (the output of the precedingadder) is designated as e and the output 2 which becomes the input ofthe phase shifter 27. The phase shifter 27 gives an output gate pulse-ata time when the controlling angle is 011 because the output of theamplifier is e as can be seen in FIGS. 4a and 4b.

Provided that the amplifiers and the phase shifter have suchcharacteristics as shown in FIGS. 4a and 4b, the operation of thecircuit of FIG. 3 will be as described hereinafter.

I. In the case of 1,, y 1,, 1,, B and 1,, I (i.e. when the transmissioncurrent 1,, is approximately equal to the set value 1,, for a relativelylong time. )2

Under these conditions, the outputs 42, 2, and 2, of the respectiveadders 15, 16 and 17 satisfy the relationships of 0 e, e, and e 0.Therefore, the outputs e e and e of the respective amplifiers 18, 19 and20 satisfy the relationships of e e and e 0. Thus, the phase shifter 27is supplied with the output e of the amplifier 18 through the diode 23to control the rectifier so as to attain the relation of 1,, I

2. In the case of I, 1,, ,8 (ie when an excessive current suddenlyflows):

Just after the realization of this state, the output I of the filter 13still remains at the level of the preceeding state. Therefore, therelationships of e e as well as O e may appear if the transmissioncurrent I, becomes suddenly large enough. In such a case, the output eof the amplifier 19 becomes larger than the output e of the amplifier18. Thus, the phase shifter 27 is supplied with the output e through thediode 24 to make the control angle a larger so as to decrease thetransmission current I,,. At this moment, the input e of the amplifier20 becomes larger and gives no influence to the phase shift controlbecause the output e of the amplifier 20 is blocked by the diode 25.

3. In the case of I, I, y (i.e. when the transmission current suddenlydecreases:

Just after the realization of this state, the output I of the filter 13still remains at the level of the preceeding state, similar to the case(2). Thus, the relationships of e 0 and e 0 are realized and therelationships of e e 0 and e e may follow to trigger the diode 25. Then,the phase shifter 27 is supplied with an input of e Since e 0, the phaseshifter 27 is supplied with an input of e by the biasing source 31 togive control pulses of a constant control angle 04 so as to quicklyrecover the normal state of the transmission.

As has been described hereinabove, when the transmission current I,changes slowly enough to ignore the time constant of a filter, theprimary control means including the filter operates to maintain constantcurrent and when the transmission current rapidly becomes too large ortoo small, an auxiliary control means operates to bring the current to anormal value.

FIG. 5 shows the comparison of the responses of a conventional controlsystem and the present control system, in which a curve 51 indicates theresponse of a conventional control system and 52 the response of thepresent control system. As is clear from FIG. 5, the flow of anexcessive current can be immediately controlled to a normal value in ashort period, according to this invention.

What is claimed is:

1. In a dc. power transmission system comprising two separate powerunits, a rectifier and an inverter actuated by said two power unitsrespectively, transmission lines connecting said rectifier and inverter,a detector for detecting the current flowing through the transmissionline, a filter for removing ripple components from the output of thedetector, and a phase shifter for controlling the control angle of therectifier to have a phase corresponding to the deviation of the outputof the filter from one setpoint, a control and protection systemcomprising at least one negative feedback system which compares theoutput of said current detector with another setpoint without theintervention of a filter to perform quick response controlling, whereinone input of a first adder is connected to the output of said filter andthe other input of said first adder receives a signal representing saidone setpoint, the output of said first adder being connected to theinput of said phase shifter, and said one negative feedback systemincluding a second adder having one input connected directly to theoutput of said current detector and a second input receiving a signalrepresentative of said another setpoint, the output of said second adderbeing connected to the input of said phase shifter.

2. The combination defined in claim 1, wherein said control andprotection system further includes an additional negative feedbacksystem comprising a third adder having one input connected directly tothe output of said current detector and a second input receiving asignal representative of a third setpoint, the output of said thirdadder being connected to the input of said phase shifter.

3. The combination defined in claim 1, wherein the output of each adderis connected to the input of said phase shifter through a respectiveamplifier and rectifier combination.

4. In a dc. power transmission system comprising two separate powerunits, a rectifier and an inverter actuated by said two power unitsrespectively, transmission lines connecting said rectifier and inverter,a detector for detecting the current flowing through the transmissionline, a filter for removing ripple components from the output of thedetector, and phase shifter means for controlling the control angle ofthe rectifier to have a phase corresponding to the deviation of theoutput of the filter from a first setpoint, the improvement comprising acontrol and protection system including a first negative feedback systemhaving means for directly comparing the output of said current detectorwith a second setpoint without the intervention of a filter to generatea first deviation signal and amplifier means for applying said firstdeviation signal to said phase shifter only when said deviation signalexceeds a prescribed value in comparison to said first setpoint, and asecond negative feedback system including additional means for comparingthe direct output of said current detector with a third setpoint withoutthe intervention of a filter to generate a second deviation signal andadditional amplifier means for applying said second deviation signal tosaid phase shifter only when said second deviation signal exceeds aprescribed valve in comparison to said first setpoint.

1. In a d.c. power transmission system comprising two separate powerunits, a rectifier and an inverter actuated by said two power unitsrespectively, transmission lines connecting said rectifier and inverter,a detector for detecting the current flowing through the transmissionline, a filter for removing ripple components from the output of thedetector, and a phase shifter for controlling the control angle of therectifier to have a phase corresponding to the deviation of the outputof the filter from one setpoint, a control and protection systemcomprising at least one negative feedback system which compares theoutput of said current detector with another setpoint without theintervention of a filter to perform quick response controlling, whereinone input of a first adder is connected to the output of said filter andthe other input of said first adder receives a signal representing saidone setpoint, the output of said first adder being connected to theinput of said phase shifter, and said one negative feedback systemincluding a second adder having one input connected directly to theoutput of said current detector and a second input receiving a signalrepresentative of said another setpoint, the output of said second adderbeing connected to the input of said phase shifter.
 2. The combinationdefined in claim 1, wherein said control and protection system furtherincludes an additional negative feedback system comprising a third adderhaving one input connected directly to the output of said currentdetector and a second input receiving a signal representative of a thirdsetpoint, the output of said third adder being connected to the input ofsaid phase shifter.
 3. The combination defined in claim 1, wherein theoutput of each adder is connected to the input of said phase shifterthrough a respective amplifier and rectifier combination.
 4. In a d.c.power transmiSsion system comprising two separate power units, arectifier and an inverter actuated by said two power units respectively,transmission lines connecting said rectifier and inverter, a detectorfor detecting the current flowing through the transmission line, afilter for removing ripple components from the output of the detector,and phase shifter means for controlling the control angle of therectifier to have a phase corresponding to the deviation of the outputof the filter from a first setpoint, the improvement comprising acontrol and protection system including a first negative feedback systemhaving means for directly comparing the output of said current detectorwith a second setpoint without the intervention of a filter to generatea first deviation signal and amplifier means for applying said firstdeviation signal to said phase shifter only when said deviation signalexceeds a prescribed value in comparison to said first setpoint, and asecond negative feedback system including additional means for comparingthe direct output of said current detector with a third setpoint withoutthe intervention of a filter to generate a second deviation signal andadditional amplifier means for applying said second deviation signal tosaid phase shifter only when said second deviation signal exceeds aprescribed valve in comparison to said first setpoint.